Prenatal Tests: What Your Patients Should Know

Kathleen Doheny

December 23, 2014

A new generation of prenatal testing options has boomed in the last few years, offering worried mothers and fathers-to-be screening tests that let them know not only the gender of their baby, but also whether the child is at risk for Down syndrome and a host of other abnormalities.

The tests — including MaterniT21 Plus, Verifi, Panorama and Harmony — are pricey if not covered by insurance, about $ 700 or more. However, they are simple and non-invasive. A blood sample from the mother is drawn and shipped off to a lab for DNA analysis.

With four companies aggressively marketing the options, competition is fierce, says Beth Daley, a reporter for the New England Center for Investigative Reporting who recently investigated the tests.

All is not rosy, she found. She uncovered some worrisome problems with the way the tests are marketed and interpreted by both patients and doctors.

WebMD asked her to discuss the highlights of her investigation and to tell us what pregnant women and their partners need to know.

Q: What did your investigation find?

A: We found evidence that some women and doctors are misinterpreting the positive results from these new screening tests. And we found in a few instances women terminating pregnancies based on the screens alone without getting confirmatory tests.

While we can never be sure why women actually go through with an abortion, the fact that they had taken these tests and gotten results that were indicative of having a child with a severe genetic disorder… .the experts think the test (results) convinced them in some way to do it.

We also found cases of many other women who almost terminated because neither they nor their providers understood a statistical blip in the way tests results are being advertised and reported.

They don't talk about what is called positive predictive value — which is basically, if you get a positive result, what are the chances that it is right? And it turns out the chance that a positive test is right is relatively small, particularly in the general population of women who are low-risk for having one of these very rare chromosomal conditions.

Q: When should women have these tests, if they decide to?

A: Probably between 10 and 14 weeks is about right.

Most women will go to their OB-GYN and be offered these tests, particularly if they are high risk and over age 35.

It's up to them and their doctor to decide (whether to undergo the screening). But the problem is, there seems to be a lack of education, again, to providers and patients about the meaning of those results when they come back. And that's where the gap is.

Doctors will say, "There is a brand new non-invasive prenatal test … it will tell you earlier if there is a problem. If there is not a problem, you can rest easily." And many times, patients agree to take the test.

The doctor collects a blood sample and sends it off to four labs, all in California, and the lab returns (results) and it says they found a chromosomal abnormality or didn't.

And this is supposed to be a risk-based test, not a diagnostics test. It is supposed to tell the woman … you have a higher risk of (your baby) having this.

Q: What do the tests look for?

A: The tests look for placental DNA that is circulating in the mother's blood. It's an amazing test, quite honestly. They do a very good job of detecting that and then testing it for chromosomal abnormalities and then reporting the results.

It is important to know they are not testing fetal DNA. They are testing placental DNA, which is not always the same.

It can detect Down syndrome, the most popular and the most accurate; Edward's syndrome, which is called trisomy 18; trisomy 13, a bunch of sex chromosome (abnormalities).

The rarer the condition in a population, the more false positives you are going to have. For example, for an older "high-risk" woman, the chances a test indicating a problem is correct is about 64%, according to a recent study by Quest Diagnostics. But for women in the general population, the chances a positive test is correct is only about 40% for Edwards syndrome, according to a well-regarded study by the testing company Illumina that was published in the New England Journal of Medicine.

Q: What are the advantages of getting these tests?

A: The tests are an incredible improvement over previous blood and ultrasound screening in early pregnancy — a 10-times-better predictor for Down syndrome, for example. This gives more women assurance earlier on their fetus does not have a problem, allowing them to avoid going for an invasive confirmatory test such as an amniocentesis that comes with a small risk of miscarriage.

Q: How should women interpret these tests, and do they get help?

A: Some doctors are very educated in maternal-fetal medicine and counsel women, "If the test comes back positive, listen, you know there actually can be a chance … that your test result could be wrong. It's an indication there are problems, it's got a high likelihood, but let's do a confirmatory test, like a CVS (chorionic villus sampling) or an amnio to be absolutely sure."

Diagnostic tests are designed to actually test for the presence of the condition. A screening test identifies a risk that you might have it — it tells you something more indirect.

Q: What kind of followup is needed?

A: You need a confirmatory test. Doctors often recommend a confirmatory test. But I believe it's all in how they recommend it. Some doctors are perfunctory, like, "You should get it confirmed." But if a doctor himself believes the tests are accurate, they may not be as forceful in telling their patients that the test can be wrong. If a doctor believes a (screening) test is 99% accurate, versus a doctor who thinks it is 50% accurate, you can imagine the conversation is very different.


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